- Asked 3/16
- 2080 Baisakh · 8 marks
- 2073 Shrawan
- 2071 Shrawan
Classes of duty by load variation, with examples
Short answer
Motor duty classes S1 to S8 define load patterns: S1 continuous (pumps, fans); S2 short-time (valve actuators, mixers); S3 intermittent periodic (cranes, presses); S4 with starting (lifts); S5 with starting and braking (mill auxiliaries); S6 continuous intermittent (pressing machines); S7 continuous with starting and braking (reversing mills); S8 with periodic speed changes (multi-speed drives).
For the selection of various types of motor, what are the classes duties to be performed by the motor on the basis of load variations? List out some examples of the driver/machine applicable to various classes of duties.
The duty of a motor is the pattern of load, rest, starting and braking it experiences with time. Since motor heating depends on this pattern, the rating must be chosen for the duty class; a motor sized for continuous duty would be oversized for short-time duty, and a short-time-rated motor would overheat on continuous duty. IEC 60034-1 defines the standard duty types; the eight classic duty classes used for motor selection are S1 to S8 (newer editions also add S9 and S10 for non-periodic and discrete-constant loads).
Classes of duty
| Class | Duty (IEC/IS) | Load pattern | Examples |
|---|---|---|---|
| S1 | Continuous duty | Constant load long enough to reach thermal equilibrium | Pumps, fans, compressors, conveyors, paper mill drives |
| S2 | Short-time duty | Constant load for a short time, then rest long enough to cool to ambient | Lock gates, bridge-opening motors, valve actuators, battery-charging motors, household mixers |
| S3 | Intermittent periodic duty | Identical cycles of load and rest; no time to reach steady temperature in either; starting heat negligible | Cranes, hoists (some), pressing and drilling machines |
| S4 | Intermittent periodic duty with starting | As S3 but starting period significant | Metal-cutting machines, lifts, hoists, cranes |
| S5 | Intermittent periodic duty with starting and electric braking | Start, run, electric brake, rest | Billet mill, rolling mill auxiliaries, centrifuges, lifts |
| S6 | Continuous duty with intermittent periodic loading | Load and no-load periods, no rest | Pressing, cutting, drilling machines, conveyor with intermittent feed |
| S7 | Continuous duty with starting and braking | Start, load, electric brake, no rest | Blooming mill, reversing rolling mill main drive |
| S8 | Continuous duty with periodic speed changes | Load at one speed, then another speed, no rest | Multi-speed drives, machine tools with gear/pole changing |
Load-time sketches
S1 continuous S2 short-time
P |______________ P |____
| | |__________ rest
+-------------> t +-------------> t
S3 intermittent S6 continuous, intermittent load
P |__ __ __ P |__ __ __
| |_| |_| |_ rest | |_| |_| |_ no-load
+-------------> t +-------------> t
Notes on each class
- Continuous duty (S1): the motor reaches its final steady temperature; rating = continuous rating (CMR). Load may be constant (fan) or variable continuous; for variable load, the equivalent (rms) current/torque/power method is used to size the motor.
- Short-time duty (S2): the motor can deliver more than its continuous rating because it never reaches final temperature; standard periods 10, 30, 60, 90 min.
- Intermittent periodic duties (S3, S4, S5): specified by cyclic duration factor (CDF) = (load time)/(cycle time), e.g. 15, 25, 40, 60 %; cycle time usually 10 min. Starting (S4) and braking (S5) losses add heating and must be included.
- Continuous with intermittent loading (S6): motor never stops; no-load periods give partial cooling.
- S7 and S8: continuous running with frequent electric braking/reversal or speed changes; heavy rotor heating, so inertia and number of operations per hour matter.
Applying to motor selection
- Identify the load cycle of the driven machine.
- Select the matching duty class.
- Find the equivalent power (rms) over the cycle and check maximum torque against motor pull-out torque.
- Choose a standard rating with that duty class marked on the nameplate (e.g. "S3 40 %").
Correct duty matching avoids both overheating (insulation damage) and over-sizing (poor efficiency and power factor).
✓ Answer checked by Claude Opus 5.5 (high effort)
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